Resonator and component with hermetic encapsulation

ABSTRACT

Proposed is a resonator which works with bulk acoustic waves and is based on a layer structure known in the art, which is arranged over a substrate. According to the invention, the total surface of the layer structure, including all resonators contained therein, is covered with a dielectric layer and a metal layer which together form an acoustic mirror, a low-k dielectric being used for the dielectric layer. The total-surface mirror offers broadband functionality over a suitable frequency range. The dielectric contained within the mirror acts as a sealing protective layer for the resonator or resonators.

The invention relates to a resonator working with bulk acoustic waves,especially a bulk acoustic wave resonator or thin-film acousticresonator (BAW resonator or FBAR resonator) and a component havinghermetic encapsulation and containing one or a plurality of suchresonators.

Such resonators are suitable in particular for bandpass filters in modemfilter technology and may, for example, be used in mobile communicationdevices.

A resonator working with bulk acoustic waves has a piezoelectric layerarranged between two metal layers (electrodes). The layers aresequentially deposited on a substrate and structured in such a mannerthat there arises a plurality of resonators, which are electricallyconnected by their correspondingly structured electrodes and maytogether realize a filter circuit, for example.

To store the acoustic energy of the bulk acoustic wave within theresonator and/or to keep acoustic energy in the substrate upon which theresonator is arranged from escaping, either an acoustic mirror isarranged underneath the resonator or an air gap is provided. An acousticmirror consists of at least two, but preferably more layers, alternatelyconsisting of materials having high and low acoustic impedance. Thematerial and the thickness of these layers are so chosen that, for theselected resonant frequency of the resonator, all layer thicknesses arein the range of a quarter wavelength (or an odd multiple of a quarterwavelength) of the acoustic wave that can propagate in the respectivematerial at this frequency. Under these conditions, the theoreticalideal case of maximum constructive interference of the acoustic wavesreflected on the boundary layers is approximately optimized and thusprevents the escape of acoustic energy from the resonator.

An air gap also serves the same purpose, because the large impedancejump between the top (or bottom) layer of the resonator and the air issufficient to reflect the acoustic wave almost completely.

A resonator working with bulk acoustic waves or a component thatexhibits such resonators is just as sensitive as a surface acoustic wave(SAW) component to mass load or to damage from contaminating substanceson the surface of the resonators. Component housings known fromsemiconductor technology in which the components are built-in, glued-in,for example, are thus normally used for such FBAR resonators andcomponents. Such housings, which consist of ceramics or metal, forexample, have two parts, consisting, for example, of a trough and a lidor a carrier and a cap, the two parts of the housing being glued,welded, or soldered together after the component has been inserted.

Such housing technologies demand high procedural and financial costs,however, and cannot keep up with the miniaturization of the componentsbecause of the required minimum dimensions, of wall thicknesses, forexample, this miniaturization being required for technological andeconomic reasons. It is therefore proposed in U.S. Pat. No. 6,087,198that the packaging be replaced by an external seal using a plasticmaterial. To prevent the plastic seal from having a negative effect onthe acoustic properties of the resonator, an acoustic mirror is providedbetween the resonator and the seal. U.S. Pat. No. 5,872,493 alsodescribes a seal, which is deposited over the component and comprises atleast one passivation layer consisting of SiO₂, an epoxy resin or adesired glob-top composition. Here too, an acoustic mirror comprising atleast three mirror layers is inserted therebetween to prevent acousticinterference with the resonator.

It is the object of the present invention to provide an encapsulationfor an FBAR resonator or a component constructed therefrom, whichsimultaneously ensures adequate hermetic sealing and adequate physicalprotection and which is more simply constructed than known solutions.

This object is achieved by a resonator having the characteristics ofclaim 1.

Advantageous embodiments of the invention are found in the other claims.

The invention proposes that an acoustic mirror, which comprises adielectric and a metal layer, be provided over a conventionally builtresonator. The dielectric layer is so designed that it represents ahermetic seal for the resonator or resonators at the same time. In thecomponent according to the invention, the metal layer that forms thesecond layer of the acoustic mirror may serve for electromagneticshielding in an advantageous manner. In comparison to the knownsolutions, a specially constructed acoustic mirror already achieves aseal in the resonator according to the invention without additionalencapsulating layers having to be arranged over the acoustic mirror.

The dielectric layer is so chosen that it alone ensures adequatehermetic sealing of the resonator or a component constructed therefrom.A resonator according to the invention is therefore much more simplyconstructed and therefore less expensive and easier to manufacture andexhibits a smaller component volume than known components.

An advantageous acoustic mirror effect is achieved when the layerthicknesses of the dielectric layer and metal layer are so chosen as afunction of the material used that their thickness corresponds toapproximately one fourth of the wavelength (or to an odd multiple of aquarter wavelength) of the bulk acoustic wave that can be propagated inthe corresponding material.

Moreover, the reflection of the acoustic mirror is affected by adifference in the acoustic impedance of the two mirror layers, thisdifference being as large as possible. In a preferred embodiment of theinvention it is therefore proposed that an organic layer, a low-kdielectric in particular, be used for the dielectric layer. These kindsof materials on electronic components are known as dielectrics and arehere proposed for the first time as a functional constituent part forcomponents working with bulk acoustic waves. These low-k dielectrics arecharacterized by an extraordinarily low acoustic impedance and usuallyalso have extraordinarily good insulating and sealing properties, makingthem particularly appropriate for the sealing mirror-layer according tothe invention.

The object of the invention is already achieved with an acoustic mirrorthat exhibits two suitable mirror layers (dielectric layer and metallayer) of low and high acoustic impedance respectively. If theencapsulation is also required to have high physical strength, then itis proposed in a further development according to the invention that atleast one other layer or another pair of layers be so arranged over theacoustic mirror that there results an alternating sequence of layers ofrelatively low acoustic impedance and layers of relatively high acousticimpedance. Since a sufficiently high acoustic impedance can already beobtained with the first two layers, namely the dielectric layer and themetal layer, both the selection of the material and the precise layerthickness are less critical for the acoustic mirror layers to bearranged thereon than in the case of the first two layers. Othermaterials, particularly less costly materials, which would be lesssuitable or unsuitable for the acoustic mirror itself comprising adielectric layer and metal layer, therefore also come into considerationfor the additional single layers or layer pairs to be deposited thereon.

The invention may be realized with a single resonator. Since usualapplications of resonators working with bulk acoustic waves are usuallyfilter circuits, a circuit comprising a plurality of interconnectedresonators may also be encapsulated with the invention. Such a circuitarrangement is normally structured out of a common layer structurecomprising at least a first electrode, a piezoelectric layer and asecond electrode. A suitable interconnection of the individualresonators, which may represent a ladder-type circuit or a latticecircuit for example, is achieved by appropriate structuring steps of theelectrode layers and possibly also of the piezoelectric layer. Such acircuit arrangement may comprise an arbitrary number of resonators. In aladder-type structure, at least two resonators are needed for a simplefilter effect. The structure may be supplemented with additionalresonators to increase the selectivity of the filter. The resonators,which are structured from the common layer structure and interconnectedtogether, are mutually covered with a dielectric and metal layer. Toprevent a capacitive coupling of the resonators across the metal layerand possibly other electrically conducting layers deposited thereon,these layers may be electrically separated on the basis of theresonators.

The layer structure is produced through the thin-film process bydepositing the single layers sequentially on top of each other on awafer, possibly arranging an acoustic mirror or other fit-inducing andgrowth-inducing layers therebetween. Such a wafer may comprise ofconventional substrate materials, especially silicon, gallium arsenide,glass, ceramics or any other substances suitable as carrier material.Due to the small sizes of resonators according to the invention or ofcomponents produced from a plurality of resonators, it is possible toproduce from one layer structure a large number of componentssimultaneously and in parallel on one wafer. It is then also possible todeposit the dielectric and metal layer total-surface over all componentsproduced on one wafer. It is moreover possible and advantageous toarrange additional active or passive circuit elements on the wafer andinterconnect them integrated with the resonators. Such active andpassive circuit elements may also be mutually covered with the sealaccording to the invention, comprising the dielectric layer and themetal layer.

After the last layer has been deposited and possibly structured, thecomponents are separated across the entire layer structure, includingthe substrate, by a sawing process, for example. There is nodisadvantage in exposing the dielectric layer on the lips of theindividual components, because it is exclusively the dielectric layerthat provides the sealing action. The metal layer serves exclusively asa layer of high acoustic impedance for the acoustic mirror and, with asuitable electrical connection, may serve as an electromagneticshielding layer.

As already mentioned, on the wafer it is possible to build circuitscomprising active and passive circuit elements in addition to theresonators, especially microstrip transmission lines, inductance coils,capacitors, transistors, diodes, and resistors. With the aid of theresonators and the additional circuit elements, it is possible toproduce circuits such as a high-frequency circuit, adaptation circuit,antenna circuit, diode circuit, transistor circuit, highpass filter,lowpass filter, bandpass filter, tuning filter, bandstop filter, poweramplifier, preamplifier, LNA, diplexer, duplexer, coupler, directionalcoupler, memory element, balun, mixer, or oscillator. No acoustic mirroris indeed needed for the other circuit and matching elements, but heretoo the dielectric layer deposited on the total surface serves as asealing layer and the metal layer serves as electromagnetic shieldingfor the circuit elements.

A desirable and attainable property for the dielectric layer is itswell-reproducible capability of being deposited in a thin-film process,including control of the layer thickness. Also desirable andadvantageous is a low dielectric constant, low water permeability, lowwater absorption and, in particular, low acoustic impedance.

All of these properties are especially advantageously realized in abenzocyclobutene. Benzocyclobutenes are known from the semiconductorindustry, under the name Cycloten®, for example, and are used inparticular as intermediate layers, dielectrics and sealing layers formicroelectronic circuits. Advantageous in particular are the lowdielectric constant and the good layer properties, especially the highlayer homogeneity that can be attained with a benzocyclobutene.

Benzocyclobutenes may be diversely substituted to accentuate orstrengthen desired material properties. Under the action of heat, theypolymerize into partially aromatic polycyclic systems, which are nearlychemically inert. In the thin-film process, benzocyclobutenes may bedeposited with high layer thickness accuracy, so that it is particularlysimple to produce a dielectric layer that is lambda-quarter thick asprecisely as possible for a component according to the invention. Theeffect of the elastic properties of the dielectric may be such that itis possible to partially or completely compensate for layer stress,which can build up on the boundary surfaces to layers lying below or tolayers deposited thereabove due to the different thermal expansioncoefficients of the layers. Dielectrics based on linked stable polymersare used as stress compensation layers for manufacturing integratedoptic components in the semiconductor industry, for example. Besidesbenzocyclobutenes, other low-k dielectrics that feature a low acousticimpedance and may be used according to the invention for the dielectriclayer and the layers of relative low acoustic impedance are known.Examples of low-k dielectrics are aerogels, porous silicates,organosilicates, a siloxane derived from condensed silsesquioxanes, apolyaromatic compound or cross-linked polyphenylene.

If these materials are used for the dielectric layer, then it is alsopossible in accordance with another embodiment of the invention to firstplanarize the dielectric layer above the resonators and additionalcircuit elements that may be present. In this process, an embeddedsurface is obtained for the dielectric layer, which, however, means thatdifferent layer thicknesses will be obtained on the dielectric layerabove the single resonators, above the wafer, or above other circuitelements. According to the invention, the component is planarized insuch a manner that the layer thickness of the dielectric layer remainingabove the resonators corresponds to a λ-quarter layer (or an oddmultiple of the λ-quarter layer). A planarized surface of the dielectriclayer has the further advantage that it substantially facilitates thefurther deposit of additional layers and in particular improves adhesionand saves material. With a planar layer deposited over a plurality ofresonators and possibly additional circuit elements, there also resultsa physically loadable surface upon which it is possible to depositadditional structures, such as another metallization plane, electricalterminal pads, solderable terminal pads, for example, permitting thedeposit of bumps that make it possible to connect the component to aprinted circuit board, a module substrate or an external circuitenvironment in flip-chip technique.

The invention will be explained in more detail below based on exemplaryembodiments and the schematic figures associated therewith.

FIG. 1 shows a resonator that has an acoustic mirror and has beendeposited on a substrate.

FIG. 2 shows a resonator deposited over an air gap.

FIG. 3 shows an encapsulation for a resonator.

FIG. 4 shows a resonator with a glob top seal.

FIG. 5 shows a resonator according to the invention.

FIG. 6 shows two resonators according to the invention.

FIG. 7 shows two resonators according to the invention with oneplanarized dielectric layer.

FIG. 8 shows the fitting arrangement having a continuous bottom acousticmirror.

FIG. 9 shows a component according to the invention having another layerpair above the metal layer.

FIG. 10 shows a component according to the invention within which onefurther circuit element is integrated.

FIG. 11 shows the simulated forward behavior of a duplexer constructedof resonators according to the invention having a mutual encapsulation.

FIG. 1 shows, in schematic cross section, an FBAR resonator known in theart, which is realized as thin-film structure SA on an arbitrarysubstrate SU. An acoustic mirror AS is provided directly above thesubstrate, possibly above adaptation layers. The acoustic mirrorcomprises at least two, preferably three layers, which are more thanλ-quarter, alternately having high and low impedance. The actualresonator, consisting of a first electrode layer ES1, a piezoelectriclayer PS and a second electrode layer ES2, is constructed above theacoustic mirror AS.

FIG. 2 shows an alternative method of manufacturing an FBAR resonatorwithout an acoustic mirror. In this case, the layer structure consistsof an electrode layer ES1, a piezoelectric layer PS and a secondelectrode layer ES2. Once the layer structure SA has been made, thesubstrate is thinned in the vicinity of the resonator, wherein either athin membrane M remains, or wherein the substrate is removed completelyand the bottom electrode layer ES1 is exposed in the vicinity of theresonator.

FIG. 3 shows, in a schematic cross section, a known method ofhermetically encapsulating an FBAR resonator. Here the conventionallybuilt layer structure SA, depicted in FIG. 1 and FIG. 2, for example, iscovered with a cap-shaped cover AD in such a manner that an air gap LSremains above the layer structure SA so that no deadening of theacoustic vibration can occur. The cover AD may be glued or soldered tothe substrate SU or fastened thereto in other ways.

FIG. 4 shows, in schematic cross section, another method for sealing anFBAR resonator known from U.S. Pat. No. 6,087,198 B, which has alreadybeen mentioned. An acoustic mirror 48 comprising at least three layersis arranged above the conventional layer structure 41. The singleresonator is next covered with a glob-top paste, such as an epoxy resinwhich is applied in liquid form and then cured. The glob-top cover ofthis single resonator is intended to protect it from environmentalexposure.

FIG. 5 shows, on the basis of a schematic cross section, parts of acomponent according to the invention. This comprises a layer structureSA, which contains the resonator and possibly the acoustic mirror, andis deposited over a substrate SU. According to the invention, the entireresonator realized in the layer structure is now covered with adielectric layer DS, whose thickness corresponds to approximately onefourth (or an odd multiple thereof) of the wavelength of the acousticwaves propagatable therein. At the same time, the dielectric layer DSserves for sealing the component and is preferably constructed of anorganic low-k dielectric.

A metal layer MS, whose thickness likewise corresponds to one-fourth ofa wavelength (or an odd multiple thereof) in the working frequency ofthe resonator is arranged thereon. The metal is specially selected fromthe viewpoint of maximum acoustic impedance. The metals tungsten,molybdenum or gold are thus particularly appropriate for the metallayer.

FIG. 6 shows, on the basis of a schematic cross section, a componentcomprising a plurality of resonators. The figure depicts two resonatorsR1, R2 structured from one layer structure and arranged on a substrateSU. A dielectric layer of thickness D1, which corresponds to a quarterof the wavelength, is deposited thereon over the total surface. Thedielectric layer is preferably deposited conformal and therefore followsthe topology of the resonators R1 and R2. This does not require that thedielectric layer DS have the same layer thickness D1 everywhere, onlythat the thickness be maintained over the resonators.

A metal layer MS, which at least over the resonators likewise has alayer thickness D2 that corresponds to one-quarter of the wavelength ofthe resonator working frequency, is arranged over the total surface ofthe dielectric layer DS. It is clearly evident from the figures that thedielectric layer, which may terminate outside the resonators on thesubstrate SU, completely covers the resonators R1, R2. The sealingcharacteristics of the dielectric layer, particularly its low waterabsorption and low water permeability and its tightness to gases withhigh molecular volumes and to liquids that are unable to penetrate thecross-linked or linked polymer network, optimally protect the resonatorfrom environmental exposure. Furthermore, the metal layer protects theresonator from physical effects and also does not affect its acousticproperties. This opens the possibility of providing additional layers,structures, metallization planes or soldered joints above the metallayer. At the same time, the metal layer MS provides for electromagneticshielding of the resonators. This is particularly advantageous forresonators used in filter circuits for front-end modules in mobilecommunications, especially in the reception unit.

The desired working frequency of the resonator is critical for the layerthicknesses D1 and D2, which are important to the invention. If theresonators are used in HF filters for the 2 gigahertz range, forexample, then there results, for benzocyclobutene as dielectric layerDS, for example, a layer thickness D1 of approximately 200 nm, whichcorresponds to a quarter of the wavelength. This value is well below thelayer thickness that can be accurately controlled by BCB(benzocyclobutene), for example. As for the layer thickness D2 of themetal layer MS, the resulting layer thickness is between 650 and 700 nmfor tungsten, for example, a thickness that can likewise be technicallycontrolled and precisely set. If higher layer thicknesses are desiredfor the dielectric layer DS and/or metal layer MS for technologicalreasons (e.g. to better cover the edges of components or to achieve ahigher air-tightness), then it is possible to resort to odd multiples ofone-quarter of the wavelength for the respective layer thicknesses.Instead of 200 nm of BCB (corresponding to approximately onelambda-quarter layer at a frequency of 2 GHz), 600 nm of BCB(approximately a 3λ/4 layer at a frequency of 2 GHz) may be deposited.The technological advantages achieved by this measure (e.g., bettercovering of the edges through increased conformity) must be preciselyweighed against potential disadvantages in acoustic performance(possibility of higher insertion loss from increased viscous losses inthicker layers).

The layer combination of a low-k dielectric and a high-impedance metallayer first proposed according to the invention has the furtheradvantage that two layers are sufficient to reflect a high percentage(more than 95%) of the acoustic energy on the boundary surfaces of thesetwo layers back into the resonator. The mirror obtains a high bandwidthfrom the small number of only two mirror layers. This means that thefrequency components lying within the mirror bandwidth can be reflecteduniformly well. To be more precise, this means that the layercombination BCB/W, for example, as acoustic reflector for frequenciesaround 2 GHz, is equally well suited for all such resonators whosefrequencies do not deviate more than approximately ±7% from a givenfrequency, even when the thicknesses of the mirror layers do not exactlyagree with the theoretical ideal value λ/4 for the correspondingacoustic wavelengths λ. Alternatively, a thickness tolerance of ±7% maybe taken into consideration in this manner for the same frequencieswithout impermissibly reducing the reflectivity of the layer pair.

The small number of only two layers for creating an acoustic mirror withhigh reflection and high bandwidth may be particularly attributed to thelow acoustic impedance of the low-k dielectric, which is approximatelyan order of magnitude lower than for conventional mirror layers havinglow impedance, especially than for SiO₂, which has been used until now.The high mirror bandwidth permits layer thickness fluctuations of ±7%.For a 200-nm layer thickness for BCB, this corresponds to maintaining aprecision of ±14 nm. This is easy to maintain, because it is possible toproduce BCB with a layer thickness accuracy of ±0.5% according to thestate of the art. Still higher layer thickness accuracies can beattained for depositing the metal layer MS. It is moreover possible,besides the layer thickness of λ-quarter, to increase the layerthicknesses to odd multiples of λ-quarter. This may be advisable for themetal layer, for example, in order to create a physically solid andsolderable surface, for example. There is no problem in thickening themetal layer because a large portion of the acoustic wave already occurson the boundary surface from resonator R or from the layer structure SAto the dielectric layer DS.

Since only a small portion of the acoustic energy is thus able topenetrate the metal layer MS at all, it is much less critical to preventreflection based on a layer thickness D2 that is inexactly set toλ-quarter, so that a 50% layer thickness accuracy for the metal layer MSis sufficient, for example. This is important, because layer thicknesstolerances must usually only be maintained as percentages, whereas theabsolute tolerance or deviation prevails for an acoustic mirror.

FIG. 7 shows, in schematic cross section, another embodiment of theinvention. It too presents a component, which comprises a plurality ofresonators R1, R2 and is produced as layer structure on a substrate SU.In contrast to the embodiment illustrated in FIG. 6, here the dielectriclayer DS is likewise deposited on the total surface, but thenplanarized. The dielectric layer thus has different layer thicknesses,because the open spaces between the resonators R1, R2 are likewisefilled with the material of the dielectric layer DS. For an appropriatematerial dimensioning and suitable or controlled process, it is alsopossible to set the thickness D1 to the desired value of λ-quarter forthe planarization of the dielectric layer above the resonators R1, R2.This facilitates the deposit of additional layers, especially the metallayer MS, over such a planarized dielectric layer DS. One standardmethod of planarizing dielectric layers such as BCB is chemicalmechanical polishing (CMP) of the surface. Here it is possible to setthe required mirror layer thickness without significantly increasing theroughness of the dielectric. Only sub-nm roughnesses arise.

FIG. 8 shows another embodiment of the invention in which one singleacoustic mirror AS may be used for a series of resonators R1, R2. In acommon filter circuit, such as a ladder-type circuit, a distinction ismade for example between resonators arranged in series and theresonators arranged in parallel branches, besides the fittingarrangement in the circuit a distinction also existing in the fact thata different resonance frequency is set. Besides a broadband acousticmirror AS, which is deposited according to the invention above theresonators in the form of dielectric layer DS and the metal layer MS, asimilar broadband mirror may similarly be provided between the substrateand the resonators. Here too, two mirror layers suffice to attainbroadbandedness for a layer combination comprising low-k dielectric andhigh-impedance metal layer in order to ensure high reflection exceeding95%. Here the broadbandedness of the acoustic mirror is used to make thesame reflectivity available for the different frequencies of paralleland serial resonators.

FIG. 9 shows, in schematic cross section, another embodiment of theinvention in which the layers are deposited over the metal layer. Inparticular, these layers may be additional low-impedance layers NI and ahigh-impedance layer HI in alternating sequence. But since thecombination dielectric layer/metal layer already possesses adequatereflection for the acoustic wave in the range of the resonancefrequency, the acoustic impedance of the additional layers LI, HI is ofonly minor importance. Preferably, however, a layer of lower impedancewill be created directly above the metal layer.

FIG. 10 shows, in schematic cross section, a component that has beenexpanded by an additional circuit element SE and which is likewisecompletely encapsulated with the dielectric layer and the metal layer.The additional circuit element SE may be an active circuit component,such as an integrated circuit, IC. In addition, the circuit element SEmay also be a passive component, such as an inductive, capacitive, orresistor element structured by metallizing. This circuit element SE maybe interconnected with the resonators R1, R2 and form an adaptationcircuit, for example. The encapsulation according to the invention makesit possible to provide arbitrary circuit elements and encapsulate themin common and to therefore also produce arbitrary circuits having theresonators R1, R2.

In a preferred exemplary embodiment, a duplexer circuit, which issuitable for the mobile radio standard UMTS, is produced according tothe invention. Both RX and TX filters are constructed from FBARresonators, which may be formed from the same layer structure SA bystructuring. The different resonance frequencies that are needed are setby additional layer deposit, by additional separating layers or bystructured cutting of a layer to the required layer thickness. Thedielectric layer is deposited over all serial and parallel resonators asa k-quarter mirror layer having a thickness of 220 nm. The acousticimpedance of BCB is equal to 1.7×10⁶ kg/sm². A tungsten layer with athickness of approximately 680 nm is deposited as a high-impedance layeror as a metal layer. Its impedance is then 94×10⁶ kg/sm². Because of thelow sensitivity of the reflection on the layer thickness of the metallayer, the same results are obtained with a layer thickness of up to 1μm.

A common acoustic mirror may be provided for all resonators. In anotherembodiment of the invention, it is possible in a suitable structuringstep, which already occurs when the layers are deposited, to separatethe electrically conducting mirror layers on the basis of resonators tobe capacitatively decoupled, particularly the metal layer and the layersof the acoustic mirror consisting of metal and located below theresonators, in order to prevent capacitive coupling between individualresonators.

FIG. 11 shows the simulated forward behavior (contributions of thecomplex transmission functions S(Ant, Rx) and S(Ant, Tx) of the complex3-port duplexer scattering matrix) of a duplexer built according to theinvention in this manner from FBAR resonators with a mutualencapsulation. It can be seen that the structure according to theinvention well satisfies the typical requirements placed on the forwardbehavior of a UMTS duplexer, for both Rx and for Tx filters.

The electrical interconnection of the resonators, which may be attainedthrough appropriate structuring of the electrode layer E1, E2 (see, forexample, FIGS. 1 and 2) in correspondence with a desiredinterconnection, such as a ladder-type circuit, is not illustrated inthe figures. The electrode layers ES may also be structured in such amanner that electrical terminal pads can be produced on the surface ofthe substrate SU outside the region stressed by the resonators. Theseterminal pads may then be made accessible either from above or below.From above, this requires the removal of the dielectric layer and metallayer and possibly other layers deposited thereon. It is also possibleto provide a feed-through through the aforementioned layers and tocompletely fill the feed-through with a conductive material, forexample. For contacting from below, feed-throughs may be provided in thesubstrate. From above, it is also possible to contact exposed terminalpads by soldering on bonding wires. It is also possible to provideintegrated wiring that connects the terminal pads to a metallizationstructure that is electrically insulated from the metal layer arrangedabove the metal layer MS. The abovementioned electrical connections,like connections through bonding wires or direct connection withflip-chip bonds, may be performed in this metallization plane. Flip-chipbonding is also possible with terminal pads or with terminal pads thatare provided directly on the substrate surface and distant from thedielectric layer DS and metal layer MS.

For the sake of clarity, the invention has only been presented exactlyon the basis of a few exemplary embodiments. But the invention is notlimited to the presented examples and may be further varied. Inparticular, such variations include additional layers or structures,different number and fitting arrangement of resonators next to or aboveeach other into SCF filters or CRF filters and/or additional circuitelements SE, which may possibly also be provided above the metal layerMS. The realizable circuits to be encapsulated with the invention arealso not limited to the described examples.

1. A resonator for use with bulk acoustic waves, the resonatorcomprising: a wafer: a layer structure above the wafer; a dielectriclayer above the layer structure, the dielectric layer comprising ahermetic encapsulation for the resonator, the dielectric layercomprising a material and having a thickness that result in a firstacoustic impedance; and a metal layer above the dielectric layer, themetal layer comprising a material and having a thickness that result ina second acoustic impedance, the second acoustic impedance being higherthan the first acoustic impedance, the metal layer and the dielectriclayer being parts of an acoustic mirror for bulk acoustic waves in theresonator; wherein the layer structure comprises: first and secondelectrode layers that comprise electrodes for the resonator; and atleast one piezoelectric layer that is between the first and secondelectrode layers.
 2. The resonator of claim 1, wherein thicknesses ofthe dielectric layer and the metal layer are in a range of a quarterwavelength of the bulk acoustic waves or in a range of an odd multipleof the quarter wavelength.
 3. The resonator of claim 1 wherein theacoustic mirror comprises at least one other layer pair arranged abovethe metal layer, the at least one other layer pair consisting comprisinga layer of relatively low acoustic impedance and a layer of relativelyhigh acoustic impedance.
 4. A component comprising: a plurality ofresonators that are electrically interconnected by electrode layers toform at least a portion of a circuit; wherein the plurality ofresonators comprise: a wafer layer structures on the wafer; a dielectriclayer above the layer structures, the dielectric layer comprising ahermetic encapsulation for the plurality of resonators, the dielectriclayer comprising a material and having a thickness that result in afirst acoustic impedance; and a metal layer above the dielectric layer,the metal layer comprising a material and having a thickness that resultin a second acoustic impedance, the second acoustic impedance beinghigher than the first acoustic impedance, the metal layer and thedielectric layer being parts of an acoustic mirror, wherein each of thelayer structures comprises first and second electrode layers thatcomprise electrodes, and at least one piezoelectric layer that isbetween the first and second electrode layers.
 5. The component of claim4, wherein the dielectric layer comprises an organic layer.
 6. Thecomponent of claim 5, wherein the dielectric layer comprisesbenzocyclobutenes.
 7. The component of claim 4, wherein the dielectriclayer is over substantially an entire surface of the wafer and over theplurality of resonators, a top surface of the dielectric layer beingsubstantially planar such that thicknesses of the dielectric layerneeded to implement the acoustic mirror are over the plurality ofresonators.
 8. The component of claim 4, further comprising: active orpassive circuit elements on or within the wafer and integrated with theplurality of resonators into one or more circuits, wherein layersforming the acoustic mirror comprise an encapsulation for the active orpassive circuit elements and the plurality of resonators.
 9. Thecomponent of claim 8, wherein the plurality of resonators and the activeor passive circuit elements comprise parts of a circuit on the wafer,the circuit comprising one or more of a high-frequency circuit, anadaptation circuit, an antenna circuit, a diode circuit, a transistorcircuit, a highpass filter, a lowpass filter, a bandpass filter, afilter having a tunable frequency, a power amplifier, a preamplifier, anLNA, a diplexer, a duplexer, a multi filter, a coupler, a directionalcoupler, a memory element, a balun, a mixer, and an oscillator.
 10. Anapparatus comprising: plural components according to claim
 4. 11. Thecomponent of claim 4, wherein the dielectric material comprises a low-kdielectric.
 12. The component of claim 4, wherein the metal layercomprises at least one of tungsten W, molybdenum Mo, gold Au or aluminumnitride AlN.
 13. The component of claim 11, wherein the low-k dielectriccomprises at least one of an aerogel, a porous silicate, anorganosilicate, a siloxane derived from condensed silsesquioxanes, apolyaromatic compound, a cross-linked polyphenylene and a polymerizedbenzocyclobutene.
 14. The component of claim 4, wherein the wafer has asurface comprising solderable contacts that are electrically connectedto the plurality of resonators or to one or more of a plurality ofactive and/or passive components integrated with the plurality ofresonators in circuits.
 15. The component of claim 4, wherein the waferhas an underside that comprises solderable connecting terminals, thesolderable connecting terminals being electrically connected to theplurality of resonators or to one or more of a plurality of activeand/or passive components integrated in circuits with the plurality ofresonators via feed-throughs in the wafer.
 16. The component of claim 4,wherein the component comprises a bulk acoustic wave resonator, astacked crystal filters, or a coupled resonator filter.
 17. Thecomponent of claim 4, wherein the acoustic mirror comprises at least oneother layer pair arranged above the metal layer, the at least one otherlayer pair comprising a layer of relatively low acoustic impedance and alayer of relatively high acoustic impedance.
 18. The resonator of claim1, wherein the dielectric material comprises a low-k dielectriccomprising at least one of an aerogel, a porous silicate, anorganosilicate, a siloxane derived from condensed silsesquioxanes, apolyaromatic compound, a cross-linked polyphenylene, and a polymerizedbenzocyclobutene
 19. The resonator of claim 1, wherein the metal layercomprises at least one of tungsten W, molybdenum Mo, gold Au or aluminumnitride AlN.
 20. The resonator of claim 1, wherein the wafer has asurface comprising solderable contacts that are electrically connectedto the resonator or to one or more of a plurality of active and/orpassive components.